I have used ready mash in the past for puting weight on my horses, somebody recommended it to me, im not sure if it is used for weight issues but it really work for my 19yr old pony and he was hard to keep weight on. i also rode him alot and it still worked, hope that helps :D  and i also use general conditioning cubes ;)

 for our old horse we had tried evrything on the mrket but the only one that working was spillers conditioning mix or something similar to that name.

I use a mix of alfa a oil, sugar beet and calm and condition for my warmblood, it is very good.  Conditioning cubes send him mental!